Disclosed is a sulfonated multiblock copolymer, which comprises a hydrophilic block (X) having a repeating unit represented by the following formula 1a, and a hydrophobic block (Y) having a repeating unit represented by the following formula 2, wherein the number (m) of the repeating unit of formula 1a in the hydrophilic block (X) and the number (n) of the repeating unit of formula 2 in the hydrophobic block (Y) satisfy the conditions of 4.ltoreq.m.ltoreq.400 and 4.gtoreq.n.gtoreq.400. An electrolyte membrane obtained from the sulfonated multiblock copolymer and a fuel cell using the electrolyte membrane are also disclosed: Ar.sup.1--O--Ar.sup.2a--O [Formula 1a] Ar.sup.1--O--Ar.sup.3--O [Formula 2]